The invention relates to a process for forming a low-sulfur motor gasoline and the product made therefrom. In one embodiment, process involves separating a catalytically cracked naphtha into at least a light fraction boiling below about 165xc2x0 F. and a heavy fraction boiling above about 165xc2x0 F. The light fraction is treated to remove sulfur by a non-hydrotreating method, and the heavy fraction is hydrotreated to remove sulfur to a level of less than about 100 ppm.
Catalytically cracked naphtha (xe2x80x9ccat naphthaxe2x80x9d) boiling in the gasoline boiling range is generally high in octane number resulting from the olefinic species contained therein. Cat naphtha may also contain sulfur impurities in sufficient quantity to warrant removal by hydroprocesssing, for example, in order to comply with product specifications and environmental regulations.
While hydroprocessing is a common method for sulfur removal, it also results in saturation of the desirable olefin species leading to a loss in cat naphtha octane number.
Some conventional processes attempt to remove the sulfur while maintaining octane number by separating the naphtha into at least a light fraction and a heavy fraction. Such processes take advantage of the observation that the olefins are concentrated in the light fraction and the sulfur in the heavy fraction. Accordingly, the light fraction is not hydroprocessed to preserve the olefin content and may be subsequently combined with the hydroprocessed heavy fraction to provide a naphtha having less sulfur without too great an octane number reduction. Unfortunately, some sulfur, such as mercaptan and thiophene sulfur, is often present in the light fraction, so further sulfur removal from the naphtha would be desirable.
At least one conventional process attempts to overcome this difficulty by treating the light fraction with a cobalt group metal chelate catalyst in an alkaline medium to oxidize the mercaptans to disulfides which are separated from the light fraction. Even so, such a treatment would not remove the thiophene from the light fraction, and further sulfur removal would therefore be desirable.
In another conventional process, reactive separation is employed to provide a relatively sulfur-free light fraction and a heavy fraction that contains monosulfides (xe2x80x9cconverted monosulfidesxe2x80x9d) resulting from the reactive conversion. While the light fraction would then contain less sulfur, the hydroprocessing of the heavy fraction must be conducted at greater severity in order to remove the converted monosulfides, which leads to an undesirable loss in octane number from olefin saturation.
Yet another conventional process seeks to restore octane number in the hydroprocessed heavy fraction by subsequently cracking the hydroprocessed fraction with an acidic catalyst such as ZSM-5. While octane number may be improved, the amount of desirable olefin species in the heavy fraction will still be diminished. Moreover, in order to avoid poisoning the acidic catalyst, the hydroprocessing is generally conducted at high severity to remove nitrogen impurities, leading to even more olefin saturation.
There is, therefore, a need for a process for forming a low-sulfur naphtha for gasoline blending from a cat naphtha without undesirable olefin saturation.
In one embodiment, the invention relates to a method for forming a low-sulfur, high-octane naphtha suitable for gasoline blending, the process comprising:
(a) separating a catalytically cracked naptha into at least a light fraction and a heavy fraction;
(b) treating the light fraction to remove sulfur at a hydrogen partial pressure of less than about 50 psig H2 to form a treated light fraction having a sulfur content less than about 150 ppm; and then
(c) treating the heavy fraction in the presence of a catalytically effective amount of a hydroprocessing catalyst under catalytic conversion condition in order to form a treated heavy fraction having a sulfur content less than about 150 ppm, based on the weight of the heavy fraction, and wherein the treated heavy fraction has at least 10% of the C6+ olefin in the heavy fraction.
In another embodiment, the invention relates to a product formed in accordance with such a process.
In another embodiment, the invention relates to a naphtha-boiling-range hydrocarbon suitable for gasoline blending, the hydrocarbon comprising olefins having at least 5 carbon atoms wherein
(i) the hydrocarbon contains no more than about 13 wt. % olefins, based on the weight of the hydrocarbon;
(ii) the percentage of olefins having at least 5 carbon atoms that are C5 and C6 olefins ranges from about 45% to about 65%; and
(iii) the hydrocarbon contains less than about 60 ppm sulfur, based on the weight of the hydrocarbon.
In one embodiment, the olefins having a carbon number of at least C5 are present in an amount ranging from about 13 wt. % to about 30 wt. %, based on the weight of the hydrocarbon, and about 25% to about 45% of the olefins having a carbon number of at least C5 are C5 olefins.